Over the last two decades retroviruses have served as important and productive objects of study. Experiments with retroviruses have yielded fundamental results in molecular oncology as well as mechanisms of information transfer and gene structure and expression in eukaryotic cells. Considering the recent recognition of retroviruses as important human pathogens, as well as the continuing revelation of non viral elements that appear to use similar mechanisms, it is certain that study of these agents on the molecular level will continue to be highly rewarding. The goals of this project are to continue our study of several fundamental aspects of retrovirus biology on the molecular level in order to obtain a deeper understanding of the virus-cell interaction. Areas of study include 1) the structure and function of the viral envelope protein and its cellular receptor; 2) structure of the major core protein precursor and its role in assembly; 3) mechanism of integration of the viral DNA into the DNA of the host cells; 4) mechanisms of expression of the provirus and dissection of signals which specify synthesis and processing of viral RNA; and 5) descriptions of the interactions of endogenous viruses with the host species. It is hoped that these studies will provide novel insights into important and unique features of these fascinating entities.